Provided are an airbag, an airbag assembly, and a folding method for an airbag. The airbag has a first end, a second end, and a third end, wherein the third end is formed by folding the airbag between the first end and the second end, the second end and the third end are free ends of the airbag, and the first end is used for fixing the airbag to a mounting position. When in a stored state, the airbag has a first rolled portion and a second rolled portion, the second end is located in the first rolled portion, the third end is located in the second rolled portion, the second end is located outside of the second rolled portion, and the third end is located outside of the first rolled portion. The airbag can be stored in a mounting space in a vehicle more efficiently.